Night vision goggles are binocular viewers that operate at very low light levels to allow the user to see, move, and work in the dark. The earliest night vision goggles designed for two-eyed operation utilized two complete optical systems: an objective lens assembly, an image intensifier tube, a collimator, and a telescope: one such system for each eye. The high cost of such a completely dual system eventually led to the development of a binocular night vision goggle having a single objective lens assembly, image intensifier tube, and collimator. See U.S. Pat. No. 4,463,252, issued Jul. 31, 1984 to Brennan, et al., for a Night Vision Goggle System; U.S. Pat. No. 4,737,023, issued Apr. 12, 1988 to Kastendieck, et al. for a Binocular Periscope Viewing Device Including Triple Prism For Image Splitting; U.S. Pat. No. 4,820,031, issued Apr. 11, 1989 to Kastendieck, et al. for a Binocular Periscope Viewing Device Including Triple Prism for Image Splitting and Elongate Head Column; U.S. Pat. No. 4,794,246, issued Dec. 27, 1988 to Kastendieck for a Universal Passive Night Vision System; and U.S. Pat. No. 4,741,608, issued May 3, 1988 to Kastendieck, et al. for a Turning Mirror for Integrating Image and Light Indicator Ray Bundles. The night vision goggle system disclosed by Brennan, et al., for example, utilizes an image splitter to reflect the image viewed by the collimator to each of a pair of afocal telescopes, one for each eyepiece of the goggle.
While image splitting techniques have been effective in reducing the cost, weight, and size of night vision goggles, an inherent disadvantage persists with current binocular night vision goggles utilizing image splitters. This is the large amount of axial chromatic aberration in the afocal telescopes of the goggles.
The conventional solution to the chromatic aberration problem is the addition of corrector lenses, typically cemented doublets, to the afocal telescope optics. Modern binocular night vision goggles, however, have greatly simplified afocal telescope optics consisting of only two lenses (an aspheric singlet objective lens and an aspheric singlet eyepiece lens) and a mirror. This design has permitted the use of a compact, lightweight eyepiece which is essential for many night vision goggle purposes. The addition of corrector lenses to the afocal telescopes would require, at a minimum, a pair of lenses or a cemented doublet for both the objective lens and the eyepiece lens. This would unacceptably increase the weight, size, and cost of the telescopes and of the goggle as a whole.
Consequently, a need exists for a binocular night vision goggle with an improved optical system that corrects axial chromatic aberration without the addition of corrector lenses to the afocal telescopes, and without significantly increasing the weight, size, or cost of the goggles.